1. Field of the Invention
This invention relates to a lithographic mask-to-wafer alignment system, particularly one employing x-ray or other beam-like imaging systems for printing replicas of patterns contained on masks onto photosensitive materials contained on semiconductor substrates such as silicon wafers. Particularly, the invention relates to the control of x-ray attenuation gases and wafer processing gases and the spacing between the mask membrane and wafer.
2. Description of the Relevant Art
U.S. Pat. No. 4,185,202 discloses an x-ray lithography system involving an x-ray source, an x-ray transmission chamber and an imaging or exposing chamber into which a prealigned mask-wafer fixture is moved into position under the source at the bottom of the exposing chamber for each exposure. The transmission chamber is maintained in a vacuum at a pressure less than about 10.sup.-6 torr while an x-ray attnuation gas such as helium is present in the imaging or exposure chamber which limits the throughput capabilities of the system. The improvement in the '202 patent is directed to having a movable (expandible) exposure chamber with a side wall which is lowered to engage the mask-wafer fixture. Helium is flowed into the chamber for each exposure at a rate of 10 liters/minute through a passageway. A separate atmosphere of inert gas, e.g., nitrogen at a rate of 2.9 liter/minute at a pressure of 5-50 psi is provided within the fixture and the zone between the mask underside and the wafer. Flushing of the zone is initially started while the fixture is still being loaded remote from the exposure chamber. The '202 patent also mentions that it may be advantageous to maintain the pressure in the exposure chamber greater than in the mask-to-wafer zone to prevent gaseous constituents in the latter being introduced to the former and contemplates addition of a small amount of inhibiting oxygen in the mask-to-wafer zone.
In the related application, a purge of x-ray attenuation gas on top of the mask membrane and wafer processing gas under the mask membrane is provided so as to provide an essentially zero pressure differential across the mask membrane to obviate any change in the gap distance between the mask membrane and wafer. It also recognizes that the membrane may bulge in the center which would introduce mask-to-wafer gap changes and resulting printing distortion and that the allowable pressure difference across the membrane is so small as to be not economically measurable by available sensors.